tee(1): increase I/O buffer size from 8KB to 64KB

tee(1): increase I/O buffer size from 8KB to 64KB

64KB strikes a good balance between space and time on today's
machines.  Buffers smaller than 64KB waste more time in userspace
traveling to and from the kernel.  Buffers larger than 64KB do I/O a
bit faster, but the performance improvements rapidly diminish at a
steep memory cost.

Discussed with millert@ and deraadt@.  Positive feedback from Geoff
Steckel.

Thread: https://marc.info/?l=openbsd-tech&m=163737586414354&w=2

ok millert@ deraadt@